1. Field of the Invention
The present invention relates to a system of transmission of multiplexed digital data according to a time frame and, more particularly, to such a transmission system wherein a plurality of component digital channels forming a resulting multiplex digital channel and conveying data digital signals and signalling digital signals may be extracted from this resulting multiplex digital channel or may be inserted therein.
2. Description of the Prior Art
Multiplexed digital data transmission systems are known in which data transmission takes place from a transmission station towards a reception station through one or several intermediate stations. In this case, some data are taken from the frame entering into an intermediate station and some other data re-inserted in the frame at the intermediate station lead out. It is well to remember here that each data channel occupies a well-defined time slot in the frame, the time slot number being defined in relation to a specific signal called frame locking signal or word which is repeated periodically in the transmitted signal. The interval between the start of a locking word and the start of the following locking word defines the frame length. For example, for the transmission of telephone channels within a PCM system at a rate of 2.048 Megabits per sec, a 125 microsecond frame divided into 32 time slots (TS) is used, and the first time slot TS.sub.o is assigned to the frame locking word, the 17th time slot TS.sub.16 is assigned to the transmission of signalling signals and the thirty others are assigned each, respectively, to a telephone channel. Each of these 32 time slots contains eight bits forming octets. The time slot TS.sub.16 contains two signalling half-octets transmitting the signalling of two telephone channels. The signalling time slots TS.sub.16 of 16 successive frames are arranged in a multiframe. The first time slot of the multiframe contains a multiframe locking signal and possibly an alarm signal, and the fifteen other time slots of the multiframe contain 30 half-octets associated with the 30 telephone channels. If TS.sub.16,i is a time slot of the multiframe (0.ltoreq.i.ltoreq.15), this time slot contains the signalling half-octets of channel i and channel (i+16).
A system of insertion and extraction into and from one or several data component channels of a multiplex data resulting channel has been disclosed my copending Application Ser. No. 595,170 filed July 11, 1975 and partially assigned to the same assignee as the present invention. It includes means to implement the phasing of the frame entering the station with a locally generated frame. This phasing requires the use of a variable delay circuit, such delay being at least equal to the duration of a frame. This delay circuit could either be constituted by a shift register or by a random access memory.
When the signals to transfer are represented only by data that are not associated with any signalling signal, the time slots TS.sub.16 are not endowed with any specific signification and all that is required is to phase-set the incident frame and the frame generated by the intermediate station. But in the case of a telephone channel transfer, it is necessary to transfer the data contained in the time slot assigned to said telephone channel as well as the signalling signals related to this specific telephone channel and contained in the assigned TS.sub.16. To do so, it is necessary to phase-in the incident multiframe composed of 16 frames and the multiframe generated by the intermediate station.
An obvious procedure would be to use for the multiframe phasing-in a variable delay circuit such as that used for the frame phasing-in. This solution would lead for instance, still in the case of a rate of 2.048 Megabits per second, to the use of a shift register with a capacity of 4096 bits which would be prohibitive.